Graphene is generally described as a one-atom-thick planar sheet of sp2-bonded carbon atoms that are densely packed in a honeycomb crystal lattice. The carbon-carbon bond length in graphene is approximately 0.142 nm. Graphene is the basic structural element of some carbon allotropes including graphite, carbon nanotubes and fullerenes. Graphene exhibits unique properties, such as very high strength and very high conductivity. Those having ordinary skill in the art recognize that many types of materials and devices may be improved if graphene is successfully incorporated into those materials and devices, thereby allowing them to take advantage of graphene's unique properties. Thus, those having ordinary skill in the art recognize the need for new methods of fabricating graphene and composite materials that incorporated graphene.
Graphene has been produced by a variety of techniques. For example, graphene is produced by the chemical reduction of graphene oxide, as shown in Gomez-Navarro, C.; Weitz, R. T.; Bittner, A. M.; Scolari, M.; Mews, A.; Burghard, M.; Kern, K. Electronic Transport Properties of Individual Chemically Reduced Graphene Oxide Sheets and Nano Lett. 2007, 7, 3499-3503 Si, Y.; Samulski, E. T. Synthesis of Water Soluble Graphene. Nano Lett. 2008, 8, 1679-1662.
While the resultant product shown in the forgoing methods is generally described as graphene, it is clear from the specific capacity of these materials that complete reduction is not achieved, because the resultant materials do not approach the theoretical specific capacity of neat graphene. Accordingly, at least a portion of the graphene is not reduced, and the resultant material contains at least some graphene oxide. As used herein, the term “graphene” should be understood to encompass materials such as these, that contain both graphene and small amounts of graphene oxide.
For example, functionalized graphene sheets (FGSs) prepared through the thermal expansion of graphite oxide as shown in McAllister, M. J.; LiO, J. L.; Adamson, D. H.; Schniepp, H. C.; Abdala, A. A.; Liu, J.; Herrera-Alonso, M.; Milius, D. L.; CarO, R.; Prud'homme, R. K.; Aksay, I. A. Single Sheet Functionalized Graphene by Oxidation and Thermal Expansion of Graphite. Chem. Mater. 2007, 19, 4396-4404 and Schniepp, H. C.; Li, J. L.; McAllister, M. J.; Sai, H.; Herrera-Alonso, M.; Adamson, D. H.; Prud'homme, R. K.; Car, R.; Saville, D. A.; Aksay, I. A. Functionalized Single Graphene Sheets Derived from Splitting Graphite Oxide. J. Phys. Chem. B 2006, 110, 8535-8539 have been shown to have tunable C/O ratios ranging from 15 to 500. The term “graphene” as used herein should be understood to include both pure graphene and graphene with small amounts of graphene oxide, as is the case with these materials.
Further, while graphene is generally described as a one-atom-thick planar sheet densely packed in a honeycomb crystal lattice, these one-atom-thick planar sheets are typically produced as part of an amalgamation of materials, often including materials with defects in the crystal lattice. For example, pentagonal and heptagonal cells constitute defects. If an isolated pentagonal cell is present, then the plane warps into a cone shape. Likewise, an isolated heptagon causes the sheet to become saddle-shaped. When producing graphene by known methods, these and other defects are typically present.
The IUPAC compendium of technology states: “previously, descriptions such as graphite layers, carbon layers, or carbon sheets have been used for the term graphene . . . it is not correct to use for a single layer a term which includes the term graphite, which would imply a three-dimensional structure. The term graphene should be used only when the reactions, structural relations or other properties of individual layers are discussed”. Accordingly, while it should be understood that while the terms “graphene” and “graphene layer” as used in the present invention refers only to materials that contain at least some individual layers of single layer sheets, the terms “graphene” and “graphene layer” as used herein should therefore be understood to also include materials where these single layer sheets are present as a part of materials that may additionally include graphite layers, carbon layers, and carbon sheets.
The unique electrical and mechanical properties of graphene have led to interest in its use in a variety of applications. For example, electrochemical energy storage has received great attention for potential applications in electric vehicles and renewable energy systems from intermittent wind and solar sources. Currently, Li-ion batteries are being considered as the leading candidates for hybrid, plug-in hybrid and all electrical vehicles, and possibly for utility applications as well. However, many potential electrode materials (e.g., oxide materials) in Li-ion batteries are limited by slow Li-ion diffusion, poor electron transport in electrodes, and increased resistance at the interface of electrode/electrolyte at high charging-discharging rates.
To improve the charge-discharge rate performance of Li-ion batteries, extensive work has focused on improving Li-ion and/or electron transport in electrodes. The use of nanostructures (e.g., nanoscale size or nanoporous structures) has been widely investigated to improve the Li-ion transport in electrodes by shortening Li-ion insertion/extraction pathway. In addition, a variety of approaches have also been developed to increase electron transport in the electrode materials, such as conductive coating (e.g., carbon), and uses of conductive additives (e.g., conductive oxide wires or networks, and conductive polymers). Recently, TiO2 has been extensively studied to demonstrate the effectiveness of nanostructures and conductive coating in these devices.
TiO2 is particularly interesting because it is an abundant, low cost, and environmentally benign material. TiO2 is also structurally stable during Li-insertion/extraction and is intrinsically safe by avoiding Li electrochemical deposition. These properties make TiO2 particularly attractive for large scale energy storage.
Another way to improve the Li-ion insertion properties is to introduce hybrid nanostructured electrodes that interconnect nanostructured electrode materials with conductive additive nanophases. For example, hybrid nanostructures, e.g., V2O5— carbon nanotube (CNT) or anatase TiO2—CNT hybrids, LiFePO4—RuO2 nanocomposite, and anatase TiO2—RuO2 nanocomposite, combined with conventional carbon additives (e.g., Super P carbon or acetylene black) have demonstrated an increased Li-ion insertion/extraction capacity in the hybrid electrodes at high charge/discharge rates.
While the hybrids or nanocomposites offer significant advantages, some of the candidate materials to improve the specific capacity, such as RuO2 and CNTs, are inherently expensive. In addition, conventional carbon additives at high loading content (e.g., 20 wt % or more) are still needed to ensure good electron transport in fabricated electrodes. To improve high-rate performance and reduce cost of the electrochemically active materials, it is important to identify high surface area, inexpensive and highly conductive nanostructured materials that can be integrated with electrochemical active materials at nanoscale.
Those having ordinary skill in the art recognize that graphene may be the ideal conductive additive for applications such as these hybrid nanostructured electrodes because of its high surface area (theoretical value of 2630 m2/g), which promises improved interfacial contact, the potential for low manufacturing cost as compared to CNTs, and high specific capacity. Recently, high-surface-area graphene sheets were studied for direct Li-ion storage by expanding the layer spacing between the graphene sheets as described in Yoo, E.; Kim, J.; Hosono, E.; Zhou, H.-s.; Kudo, T.; Honma, I. Large Reversible Li Storage of Graphene Nanosheet Families for Use in Rechargeable Lithium Ion Batteries. Nano Lett. 2008, 8, 2277-2282. In addition to these studies, graphene has also been used to form composite materials with SnO2 for improving capacity and cyclic stability of the anode materials as described in Paek, S.-M.; Yoo, E.; Honma, I. Enhanced Cyclic Performance and Lithium Storage Capacity of SnO2/Graphene Nanoporous Electrodes with Three-Dimensionally Delaminated Flexible Structure. Nano Lett. 2009, 9, 72-75.
While these results were promising, they fell short of producing materials exhibiting specific capacity approaching the theoretical possibilities. For example, while it has been shown that graphene may be combined with certain metal oxides, the graphene materials in these studies fall far short of the theoretical maximum conductivity of single-sheet graphene. Further, those having ordinary skill in the art recognize that the carbon:oxygen ratio and the specific surface area of graphene provide an excellent proxy to measure the relative abundance of high conductivity single-sheets in a given sample. This is because the C:O ratio is a good measure of the degree of “surface functionalization” which affects conductivity, and the surface area conveys the percentage of single-sheet graphene in the synthesized powder.
Accordingly, those having ordinary skill in the art recognize that improvements to these methods are required to achieve the potential of using graphene nanostructures in these and other applications. Specifically, those skilled in the art recognize the need for new methods that produce nanocomposite materials of graphene and metal oxides that exhibit greater specific capacity than demonstrated in these prior art methods.
The present invention fulfills that need, and provides such improved composite nanostructures of graphene layers and metal oxides that exhibit specific capacities heretofore unknown in the prior art. The present invention further provides improved and novel methods for forming these composite nanostructures, and improved and novel devices that take advantage of the new and unique properties exhibited by these materials. The present invention meets these objectives by making nanostructures of graphene layers and metal oxides where the C:O ratio of the graphene layers in these nanostructures is between 15-500:1, and preferably 20-500:1, and the surface area of the graphene layers in these nanostructures is 400-2630 m2/g, and preferably 600-2630 m2/g, as measured by BET nitrogen adsorption at 77K. While those having ordinary skill in the art have recognized the desirability of having C:O ratios and surface areas this high in the graphene of nanostructures of graphene and metal oxides, the prior art methods have failed to produce them.